In a casing or a chassis for an electronic device, multiple printed circuit boards having mounted thereon electronic components that generate heat at the time of operation are often stored in a single-layer or multiple-layer manner. An increase in temperature of the electronic device due to heat generated by the electronic components causes an operational abnormality of the electronic device. Accordingly, in the electronic device, cooling air is usually made to flow through the electronic components that generate heat in order to cool the electronic components.
As illustrated in FIG. 11B, in a casing 100c for an electronic device, the printed circuit board 200 is conventionally arranged such that the printed circuit board 200 is parallel to a first side surface plate 103 of the casing 100c. In this arrangement, cooling air, which flows from an air intake surface 126 of a front surface plate 115 of the casing 100c for the electronic device, changes, first, its flow direction to the vertical direction with respect to the first side surface plate 103 of the casing 100c. The reason for this change is that a electronic component such as a dual inline memory module (DIMM) 201a arranged on the printed circuit board 200 are arranged perpendicularly with respect to the first side surface plate 103 such that the DIMM 201a does not act as a barrier against the flow of the internal air. After the cooling air cools the electronic components on the printed circuit board 200, the cooling air changes its flow direction so as to be parallel to the first side surface plate 103. Then, the cooling air is discharged through an air discharge surface 127 of a back surface plate 105 of the casing 100c. 
Furthermore, there is a conventionally used method for arranging, in a casing for an electronic device, two electronic units, in which multiple printed circuit boards having mounted thereon the electronic components are arranged in parallel, are arranged in a V-shaped manner by inclining the electronic units with respect to the normal line of a base plate of the casing for the electronic device. In this method, a channel is formed such that cooling air from the base plate of the casing flows from the lower area to the upper area of the inclination of the printed circuit boards arranged in the electronic unit and is discharged from a top plate of the casing.
Patent Document 1: Japanese Laid-open Patent Publication No. 59-003998
However, with a conventional technology represented by the technology described above, because cooling air does not make a channel through which air flows smoothly over the printed circuit boards, there is a problem in that the cooling efficiency of the electronic components mounted on the printed circuit boards is low. Furthermore, because air flow space is needed in the casing, there is also a problem in that the size of the casing itself becomes large.
Specifically, as illustrated in FIG. 11B, the printed circuit board 200 is arranged in the casing 100c and a channel for air inflow and outflow is formed, as indicated by the arrow in FIG. 11B, the cooling air flowing into the casing 100c significantly changes its flow direction until it flows outside of the casing 100c. This causes resistance against the flow of the cooling air and the amount of the cooling air decreases, thus reducing the cooling efficiency.